This document describes a tunable resonator, and more particularly to production of electrical components for electrical circuits, specifically for precision Radio Frequency (RF) applications.
Conventional resonators are made up of an inductor and a capacitor. They can create a series circuit or shunt circuit as shown in FIG. 1. The resonant frequency of such resonator is:
                                          f            o                    =                      1                          2              ⁢                              π                ·                                                      L                    ·                    C                                                                                      ,                            (        1        )            
where
f0, is a frequency of free oscillation,
L is inductance value of the inductor,
C is capacitance value of the capacitor.
The prior art of the tunable resonator uses either a tunable inductor or a tunable capacitor, but never both. As seen from formula (1), when inductance or capacitance of the resonator changes, the resonant frequency changes as well.
Certain applications of a tunable resonator, such as a tunable filter, for example, require the resonator to have a constant bandwidth even when central frequency of the filter changes. In order to keep the bandwidth constant, the resonators must be able to change their resonant frequency while keeping a characteristic impedance constant. The resonator's characteristic impedance can be expressed as:
                                          Z            o                    =                                    L              C                                      ,                            (        2        )            
Formulas (1) and (2) demonstrate that when only one parameter changes, either L or C, the central frequency changes, yet the impedance changes as well. This is a main weakness of the prior art of tunable resonators. The change of impedance results in distortion to the frequency response curve and change of the bandwidth when tuned over a frequency range, as shown in FIG. 2. This limits the tunable frequency range to a narrow band where the distortion could be tolerated.